Process: Added initial support for process groups.

kwsysProcess_SetOption now allows you to specify a new
kwsysProcess_Option_CreateProcessGroup option, which creates the
process in a new process group (Windows/UNIX) and a new session
(UNIX).  Child process groups receive signals separately from their
parents.  This allowed for the introduction of the new
kwsysProcess_Interrupt function, which allows one to safely request
the child process in its own group to terminate.  The Ctrl+C handler
also manually sends that signal to child process groups, since it's
no longer automatic.

Change-Id: Id0a420ad65f1b1c1d299ac0eb95fbb8b50a52409